1. Field of the Invention
The present invention relates to a method for controlling water quality in a nuclear reactor, which permits to reduce concentration of cobalt radioactivity of nuclear reactor water in a nuclear power plant, as well as an amount of cobalt radioactivity transferred to an outer surface of a reactor core, and to a nuclear power plant having material, which has been changed to stainless steel effective in control of the water quality mentioned above.
2. Description of the Related Art
Many kinds of quality controlling methods in the nuclear power plant have been proposed and put into practice in order to reduce concentration of cobalt radioactivity of nuclear reactor water in the nuclear power plant, as well as an amount of cobalt radioactivity transferred to an outer surface of the reactor core. Principal methods will be described below.
There has been a method for adding iron into supply water (at an amount of at least 0.3 to 0.5 ppb at iron concentration in the supply water) to make an amount of iron, which is carried into the nuclear reactor and corrosively eluted from structural material within the nuclear reactor into nuclear reactor water, at least twice as much as any one of an amount of nickel, which is carried into the nuclear reactor, and an amount of nickel, which is generated in the nuclear reactor, thus reducing concentration of cobalt radioactivity in the reactor water.
According to the above-mentioned conventional method, it is possible to reduce concentration of ionic cobalt radioactivity in the reactor water. The conventional method however has problems of acceleration of transfer velocity of ionic cobalt radioactivity transferred outside the reactor core as well as adverse increase in concentration of radioactivity due to granular material. In addition, use of material for coated fuel pipes, which has been subjected to a specific surface treatment to provide an excellent corrosion resistance, leads to an enhanced extent of elution of radioactivity from particles adhering on the surface of the coated fuel pipes and a lower reduction ratio of concentration of ionic cobalt radioactivity in the reactor water. As a result, there has not been expected an effect of reduction in ionic cobalt radioactivity transferred outside the reactor core.
There is known a zinc injection method as a water quality control method for reducing transfer of cobalt radioactivity to the outside surface of the reactor core. According to such a zinc injection method, activation however generates radioactivity of Zn-65 with the result that natural zinc cannot be used and zinc from which the Zn-65 has been removed by means of a high-priced centrifugal separator must be used in a large amount.
In view of such circumstances, there has been developed an extremely low iron condition operation method, which permits to reduce remarkably transfer velocity of ionic cobalt radioactivity transferred outside the reactor core, although concentration of ionic cobalt radioactivity in the reactor water slightly increases. According to such a method, it is possible to reduce remarkably adhesion of particles on the surface of coated fuel pipes, resulting in remarkable reduction in concentration of radioactivity due to particle material in a nuclear reactor residual heat removal system (hereinafter referred to as the “RHR system”) for removing residual heat upon a reactor shutdown. Such a method however has a problem of impossibility of reducing radioactivity in a fuel exchange pool and the like, due to the fact that particles adhering on the surface of the coated fuel pipes easily tend to peel off.